One common use of a semiconductor laser device is to record data onto or read data from an optical storage medium such as an optical disc. One problem encountered in this use is that, when reading data from or recording data to an optical disc, a fraction of the light emitted by the laser is reflected by the optical disc and is coupled back into the laser. The reflected light and the laser beam interfere optically, and this interference cause instabilities in the laser cavity which result in the generation of noise in the laser's output power.
One proposal for overcoming this problem is to use an optical arrangement which minimises the amount of light coupled back into the laser by separating the illumination optics from the collection optics. However, this leads to a much heavier, larger and more expensive optical arrangement.
Another proposal for overcoming this problem is to drive a laser according to a method known as the “high frequency overlapping method”. In this method, a current oscillating at a high frequency is superimposed on the driving current of a semiconductor laser device. The high-speed modulation caused by the high frequency current destroys the phase coherence between the oscillating modes of the laser and any light reflected back to the laser by the optical disc, thereby rendering the laser insensitive to the optical feedback. This method has the disadvantage, however, that extra circuitry is required to bias and modulate the laser diode at a high frequency and this complicates the overall system and makes it much more expensive. Also, this method does not allow the miniaturisation of an optical set-up that uses a semiconductor laser.
Another approach to solving the problem has been to use a self-pulsating laser diode that use a direct current (DC) unmodulated drive current. Such a laser device offers low noise characteristics for optical disc reading/writing systems, by reducing the relative intensity of noise arising from the optical feedback from an optical disc.
The basic principle of a self-pulsation semiconductor laser is that the laser structure contains a layer or region that is absorbing for light emitted by the active region of the laser. Initially carriers are confined in the absorbing layer/region and are allowed to accumulate in the absorbing layer/region after it absorbs light generated in the active region. The accumulation of carriers in the absorbing layer/region causes a drop in its absorption coefficient i.e., the absorbing saturates. This reduces the loss in the cavity of the laser device, and so leads to the sudden onset of a strong laser pulse as the photon density in the cavity rises above the threshold for laser action, The device quickly stops lasing as the intense lasing mode rapidly depletes the carriers in the active region, replenishing the absorption coefficient of the laser cavity to its original value. This cycle repeats itself, and hence self-pulsation is achieved.